It is well known in the art to provide an electrical distribution system for energizing a plurality of electric circuits in accordance with a predetermined delay pattern and one such system comprises a conventional street lighting system, particularly a sodium light system, and wherein a plurality of electrical circuits, each associated with a lamp, are activated successively in accordance with a predetermined delay pattern so that the lights are illuminated in the predetermined delay sequence to avoid an overload of the local electrical distribution system. In such, a system the predetermined delay pattern is predetermined by a clock, the clock is set to actuate the light circuits in accordance with the predetermined delay program, and in such a system the actual delay between successive lights being illuminated is not important, subject to the lights being illuminated in the desired order.
The present invention is not concerned with such clock controlled electrical delay systems and such clock controlled systems are disclaimed herefrom.
In other electrical systems, for example electrical systems for igniting detonators in an explosives firing circuit in accordance with a predetermined pattern, there are factors which require that the detonators be ignited in accordance with a relatively accurate delay pattern.
One arrangement for obtaining an accurate time delay sequence for a plurality of circuits is disclosed in the British Patent Publication No 2015791B and wherein an accurate pulse generating means is provided in a control unit and a relatively cheap, and relatively inaccurate, pulse generating means is provided in each of the delay circuits associated with the respective loads. The accurate pulse generator in the control unit sends relatively accurate timing interval signals and each electrical load circuit includes means for receiving the timing interval signals from the control unit, means for allowing the pulse generator of each delay circuit to run for the time interval defined by the control unit and for counting the number of pulses made by the pulse generator during the time interval signals means. The delay circuits are arranged to calculate a further time interval as a function of the said time interval and, on receipt of a fire signal, each pulse generating means is then arranged to run for said time interval and said function of the time interval before activation the relevant electrical load circuit.